assign fields dynamically in newforms

DynamicFieldSnippetForm demonstrates how to dynamically assign fields in newforms.

1. weight is a required static field
2. height is an optional dynamic field

This example uses request_height as an optional keyword argument to declare whether the height field should be added to the form, but it's just there for demonstration purposes. If you decide to use a keyword argument in your code, be sure to pop it off (as demonstrated in the code) or you'll get an unexpected keyword argument error.